Mechanical ventilatory support is widely accepted as an effective form of therapy and means for treating patients with respiratory failure. Ventilation is the process of delivering oxygen to and exhaling carbon dioxide from the lungs. When receiving ventilatory support, the patient becomes part of a complex interactive system which is expected to provide adequate ventilation and promote gas exchange to aid in the stabilization and recovery of the patient. Clinical treatment of a ventilated patient often calls for monitoring a patient's breathing to detect an interruption or an irregularity in the breathing pattern, for triggering a ventilator to initiate assisted breathing, and for interrupting the assisted breathing periodically to wean the patient off of the assisted breathing regime, thereby restoring the patient's ability to breathe independently.
A patient whose breathing is being supported by a ventilator typically receives breathing gas through a ventilator conduit. The ventilator conduit generally consists of two flexible conduits, an inhalation conduit and an exhalation conduit, connected to a wye fitting. The free ends of the conduits are attached to the ventilator so that the inhalation conduit receives breathing gas from the ventilator's pneumatic system and the exhalation conduit is attached to an exhalation valve, permitting exhalation to the atmosphere. The wye fitting is typically connected to the patient's breathing attachment, which is oftentimes an endotracheal tube, which conducts breathing gas into the lungs of the patient, and exhaled gas from the lungs of the patient to the exhalation conduit.
In those instances where a patient requires mechanical ventilation due to respiratory failure, a wide variety of mechanical ventilators is available. Most modern ventilators allow the clinician to select and use several modes of inhalation either individually or in combination. These modes can be defined in three broad categories: spontaneous, assisted or mechanically controlled. During spontaneous ventilation without other modes of ventilation, the patient breathes at his own pace, but other interventions may affect other parameters of ventilation including the tidal volume and the baseline pressure within the system. In assisted ventilation, the patient initiates the inhalation by lowering the baseline pressure, and then the ventilator “assists” the patient by completing the breath by the application of positive pressure. During mechanically controlled ventilation, the patient is unable to breathe spontaneously or initiate a breath, and is therefore dependent on the ventilator for every breath.
Regarding intubated patients receiving ventilator support, resistance and work of breathing are measured because these parameters are essential for correct bedside patient assessment and for evaluating the effects of ventilator therapy. Respiratory resistance is the amount of pressure required to deliver a given flow of gas and is expressed in terms of a change in pressure divided by flow. Total respiratory resistance (RTOT) is the sum of physiologic airways resistance (RAW) and endotracheal tube resistance (RETT) (i.e., RTOT=RAW+RETT). The standard units of resistance are cm H20/L/second.
Bronchodilator therapy is widely used in mechanically ventilated patients with severe asthma and/or chronic obstructive pulmonary disease. Studies have demonstrated that mechanically ventilated patients, including some patients without a previous diagnosis of airway obstruction, have improvement in their expiratory airflow after bronchodilator administration (Gay P. C. et al., “Evaluation of bronchodilator responsiveness in mechanically ventilated patients,” Am. Rev. Respir. Dis. 136:880-885 (1987)). Previous studies have not been able to distinguish the individual contribution of RAW and RETT to RTOT; however, this information could enable clinicians with new tools to improve the level of care for the patient. For example, increased RTOT secondary to abnormally increased RAW indicates the need for bronchodilator treatment, while increased RTOT as a result of increased RETT indicates the need to evaluate the patency of the endotracheal tube or to suction it and not administer bronchodilator treatment.
Bronchodilator administration is given to many patients, some of which administration is effective in opening patient airways, decreasing airway resistance and generally making it easier for the patient to breathe. Bronchodilators given in excess may cause unwanted side effects such as nervousness, restlessness, trembling, and dry mouth. Previous inventors have taught that by optimizing bronchodilator administration of the time and duration of application one can reduce the amount of unneeded bronchodilation therapy administered, US 2005/0284469 A1. To date, clinicians do not measure at the patient's bedside either physiologic airway resistance or endotracheal tube resistance because these parameters are not intuitively measurable. Currently, only total respiratory resistance is measured or estimated at the bedside. The conventional method of measuring total respiratory resistance requires a clinician to temporarily interrupt the patient's breathing and apply an end inspiratory pause. The clinician then has to carefully measure pressures and flows and perform calculations by hand. This unwieldy method provides an assessment of RTOT only, and does not account for RAW and RETT. Moreover, this method of determining respiratory resistance is impractical for spontaneously breathing patients, such as those receiving pressure support ventilation (PSV) and intermittent mandatory ventilation (IMV).
In addition, without measuring RAW and RETT, appropriate ventilatory therapy may be compromised and therapeutic resources squandered. Presently, clinicians routinely implement bronchodilator breathing treatments upon increased RTOT. Unfortunately, in the inventor's experience, as much as 25% or more of those instances do not require the treatment because RAW is not increased. Rather, RETT is increased and the proper strategy would have been to evaluate the patency of the endotracheal tube. Thus, at least 25% in resources and monetary savings associated with bronchodilator treatments can be recouped if RAW and RETT could be monitored for ventilator-dependent patients.
The total work of breathing (work to initiate and sustain a breath, “WOBTOT”) performed by a patient's inspiratory muscles to inhale while intubated and attached to the ventilator may be divided into two major components: physiologic work of breathing (WOBP) and breathing apparatus (endotracheal tube and ventilator) imposed resistive work of (WOBI). The total work of breathing (i.e., WOBTOT=WOBP+WOBI) can be measured and quantified in joules/min.
WOBTOT with WOBP and WOBI information are important for identifying physiologic and imposed factors influencing changes in work of breathing or the loads on the inspiratory muscles to spontaneously inhale. For example, increased WOBTOT secondary to abnormally increased WOBP indicates the need to apply increased ventilatory support to unload the inspiratory muscles.
Conventional methods of measuring work of breathing require a clinician to insert a special esophageal balloon catheter, use special equipment and perform accurate bedside calculations. Specially trained personnel are needed. This is also an unwieldy method that provides limited information as it is an assessment of WOBTOT only and provides no information regarding WOBP and WOBI.
When patients are evaluated for extubation, work of breathing is assessed. If WOBTOT is abnormally increased, most physicians may conclude the patient should remain intubated. Unfortunately, this does not take into account either WOBI or WOBP. Where it is determined that WOBTOT is increased due to increased WOBI, and WOBP is normal, then the patient may be extubated, saving the hospital and patient the cost of a ventilator for another day.
The early generation of mechanical ventilators, prior to the mid-1960s, was designed to support alveolar ventilation and to provide supplemental oxygen for those patients who were unable to breathe due to neuromuscular impairment. Since that time, mechanical ventilators have become more sophisticated and complicated in response to increasing understanding of lung pathophysiology. In an effort to improve a patient's tolerance of mechanical ventilation, assisted or patient-triggered ventilation modes were developed. IMV, a method of ventilatory support that supplements spontaneous ventilation, became possible for adults outside the operating room in the 1970s. Varieties of “alternative” ventilation modes addressing the needs of severely impaired patients continue to be developed.
In recent years, microprocessors have been introduced into modern ventilators. Microprocessor ventilators are typically equipped with sensors that monitor breath-by-breath flow, pressure, and volume, and derive respiratory parameters. Their ability to sense and transduce “accurately,” combined with computer technology, makes the interaction between clinician, patient, and ventilator more sophisticated than ever. The prior art microprocessor controlled ventilators suffered from compromised accuracy due to the placement of the sensors required to transduce the data signals. Consequently, complicated algorithms were developed so that the ventilators could “approximate” what was actually occurring within the patient's lungs on a breath-by-breath basis. In effect, the computer controlled prior art ventilators were limited to the precise, and unyielding, nature of the mathematical algorithms that attempted to mimic cause-and-effect in the ventilator support provided to the patient.
U.S. Pat. No. 5,316,009, which is incorporated herein by reference, describes an apparatus for monitoring respiratory muscle activity based on measuring resistance and elastance of the lung and then calculating a value called respiratory muscle pressure (PMUS) from the equation:
      P    APPLIED    =                    P        VENTILATOR            +              P        MUS              =                  (                              R            TOT                    ·          Flow                )            +                        V          T                          C          RS                    where CRS is respiratory system compliance and VT is the tidal volume. A problem with the method taught by the '009 patent is that Pmus is difficult to measure in a spontaneously breathing patient because the parameters RTOT and Crs must be very accurately computed in order for Pmus to correlate with “work”. Moreover, RTOT and Crs in a spontaneously breathing patient with ventilator support are very difficult to obtain accurately.
Airway occlusion pressure for 0.1 seconds after breath initiation by a patient (P0.1) has also been proposed as an indicator of work of breathing. P0.1 can be based on esophageal pressure or airway pressure. An esophageal pressure P0.1 is invasive but correlates fairly well with work of breathing. An airway pressure P0.1 is non-invasive, but does not correlate nearly as well with work of breathing.
U.S. Pat. No. 5,752,921, which is incorporated herein by reference, describes an apparatus for determining tracheal pressure based on an inflatable cuff located on an endotracheal tube. Unfortunately, this patent provides no description or suggestion regarding the specific component variables of RAW, RETT, WOBI and WOBP for respiratory resistance and work of breathing, respectively, let alone how one would utilize the measured tracheal pressure to determine these specific component variables. Further, the patent requires the patient be subjected to an end-inspiratory and end-expiratory pause, which is not ideal for patient treatment, to accurately obtain pressure cuff measurements and applies to only one form of ventilatory support, controlled mechanical ventilation. Additionally, this patent does not teach that this method of determining tracheal pressure can be used for spontaneously breathing patients receiving ambient pressure or forms of positive pressure ventilation.
A number of other patents exist for respiratory systems including U.S. Pat. Nos. 6,439,229; 6,390,091; 6,257,234; 6,068,602; 6,027,498, 6,019,732; 5,941,841; 5,887,611; 5,876,352; 5,807,245; and 5,682,881, all of which are incorporated herein by reference.
Accordingly, there is a need in the art for a method and system to noninvasively and automatically monitor resistance and work of breathing, particularly RAW, RETT, WOBP and WOBI, in a ventilator-dependent patient. Furthermore a device to automatically deliver bronchodilator therapy based upon the patient's airway status and/or to monitor the effectiveness of a patient's response receiving bronchodilator therapy would prove to be novel in the art. The present invention is designed to address this need.